Consider the following information from the mass spectrum of an unknown compound.(M) +• at m/z = 136, relative height = 65.6%(M+1) +• at m/z = 137, relative height = 6.4%Propose a formula for the moleculeSelect answer from the options belowC9H12O2C9H12OC10H12OC12H9OC10H14O2
Question
Consider the following information from the mass spectrum of an unknown compound.(M) +• at m/z = 136, relative height = 65.6%(M+1) +• at m/z = 137, relative height = 6.4%Propose a formula for the moleculeSelect answer from the options belowC9H12O2C9H12OC10H12OC12H9OC10H14O2
Solution
The mass spectrum data provided indicates that the molecular ion (M) +• has a mass-to-charge ratio (m/z) of 136 and a relative height of 65.6%. The (M+1) +• ion has a m/z of 137 and a relative height of 6.4%.
The (M+1) +• peak in a mass spectrum is typically due to the presence of Carbon-13 (C-13) isotopes in the molecule. The natural abundance of C-13 is approximately 1.1%, which means that about 1.1% of all carbon atoms are C-13. The rest are Carbon-12 (C-12).
The relative height of the (M+1) +• peak compared to the (M) +• peak can give us an estimate of the number of carbon atoms in the molecule. In this case, the relative height is 6.4/65.6 = 0.0976, or approximately 9.76%.
If we divide this percentage by the natural abundance of C-13 (1.1%), we get an estimate of the number of carbon atoms in the molecule. In this case, 9.76/1.1 = 8.87, which we can round to 9.
Therefore, the molecule likely contains 9 carbon atoms.
Looking at the options provided, we can eliminate C10H12O, C12H9O, and C10H14O2 because they contain more than 9 carbon atoms.
Between C9H12O2 and C9H12O, we can use the m/z of the molecular ion (M) +• to determine the correct formula. The m/z of 136 corresponds to the total mass of the molecule.
The molecular mass of C9H12O2 is 9(12.01) + 12(1.01) + 2(16.00) = 152.17, which does not match the m/z of 136.
The molecular mass of C9H12O is 9(12.01) + 12(1.01) + 16.00 = 136.17, which matches the m/z of 136.
Therefore, the formula of the molecule is likely C9H12O.
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